As a kind of pipe joint, there is known a separation preventing pipe joint, wherein a socket formed on an end of one of the pipes to be joined together receives a spigot formed on an end of the other pipe and a separation preventing function is applied between the socket and spigot. In this pipe joint having such separation preventing function, the intended separation preventing function is exerted in that the ridge formed on the outer periphery of the front end of the spigot engages a lock ring received in the inner periphery of the socket. The ridge on the front end of the spigot is formed by forming an annular groove in the outer periphery of the front end of the spigot and fitting a separation preventing ring in this annular groove. In the case where such ridges are formed in a pipe manufacturing plant, the formation of the ridge on a pipe is attained by forming the outer periphery of the front end of the spigot with an annular groove, fitting a separation preventing ring in said annular groove, welding the ring fitted in the annular groove to the pipe, and applying a tapering process to the front end of the spigot including the front end of said ring.
However, such operation of welding the ring to the spigot presents a problem that it requires much time and labor even when the operation is performed in the manufacturing plant as described above. Further, in the case where pipe cutting operation is to be effected in order to reduce the length of pipes. e.g., in a pipe laying site, welding operation is required in such laying site; thus, there is a problem that the operating efficiency is very low.